Method and apparatus for drying compressed gases

ABSTRACT

Compressed gases are dried by heat exchanging them with a refrigerating fluid in a closed refrigeration circuit. The chilled gases are then reheated to use temperature by being heat exchanged with condensing refrigerating fluid. The refrigerating fluid is also heat exchanged with ambient air in a condenser connected in parallel or in series with the compressed gas reheater. Color indicating means may be used in conjunction with the reheater for indicating product gas humidity and dryer performance.

United States Patent Lowell E. iinflman Wilmington, Del. 799,563

Feb. 17, 1969 June 22, 1971 Deltech Engineering, Inc. New Castle, Del.

Inventor Appl. No. Filed Patented Assignee METHOD AND APPARATUS FOR DRYING COMPRESSED GASES 11 Claims, 4 Drawing Figs.

US. Cl. 62/93, 62/90, 62/173, 62/128 Int. Cl. F25d 17/06 Field otSearch 62/93, 173, 90, 128

References Cited UNITED STATES PATENTS 1.853.,236 4/1932 Shadle 62/93 2,713,995 7/1955 Arkoosh 62/173 3,041,842 7/1962 Heinecke 62/93 3,258,932 7/1966 Kern 62/93 3,469,412 9/1969 Giufi're 62/173 Primary Examiner-William J, Wye Attorney-Synnestvedt and Lechner ABSTRACT: Compressed gases are dried by heat exchanging them with a refrigerating fluid in a closed refrigeration circuit. The chilled gases are then reheated to use temperature by being heat exchanged with condensing refrigerating fluid. The

refrigerating fluid is also heat exchanged with ambient air in a condenser connected in parallel or in series with the compressed gas reheater. Color indicating means may be used in conjunction with the reheater for indicating product gas humidity and dryer performance.

PATENTEU JUN22 I971 SHEET 1 BF 2 8 u w M mm r w I x A 2 a w 6 MW H I M N WL 3* 7 w: M n, NV 7 C. m 5 w w 7 s M 3 a w 2 mm P i m W a a X r will m o 0 M 2 WWW 1/. N @'M mfl m P A Hf 7 A W E M A R R M r MST w 5% METHOD AND APPARATUS FOR DRYING COMPRESSED GASES This invention relates to compressed gas dryers, particularly compressed gas dryers of the refrigerated type. It is concerned with improvements, both method and apparatus, in dryers of this type, which result in novel dryer systems of greater efficiency, compactness of structure, convenience of construction, and flexibility of operation.

Compressed gases, particularly compressed air, are often, in fact even normally, heavily moisture laden after compression. The high moisture content of the gases interferes in varying degrees with various intended uses of the gases and for this reason it has become an accepted practice to dry, or dehumidify, the gases before they are used. Among the broad general types of drying apparatus, is that of refrigerated air dryers. Such dryers remove the water from the compressed gas by dropping its temperature to a selected level below the dew point, thus causing the water vapor in the gas to condense into liquid water and fall out of the gas. The product gas is always saturated with water vapor at the lowest operating temperature of the dryer, but is far below saturation, and hence sufficiently dry, at higher temperatures where it is normally used.

It is known in the art that the cooling of the compressed gas can be accomplished by heat exchanging it with a refrigerating fluid such as the common Freons, the refrigerating fluid being confined in a normal refrigeration circuit usually including a compressor, condenser, expansion valve or the like, receiver, and, of course, the evaporator-chiller itself.

In such a simple system, the product gas is cold, which is of no advantage in many instances, and can even be disadvantageous. The most common fonns of using equipment for compressed gas do not require that it be cold, and since moisture tends to condense from the atmosphere on cold surfaces, the gas carrying lines will sweat" and drip, which can interfere with, and corrode, or eventually destroy operating equipment.

The foregoing circumstances, combined with the fact that the heat absorbing capacity of the cold product gases is itself of utility, have led prior workers to suggest various systems for either increasing the efficiency of the refrigerated gas dryer or for otherwise making use of the coldness of the dried gas, before it is used for its principal intended purpose. One use that has been made of the cold product gases is to precool the incoming product gases, through a heat exchanger. This approach, if carefully applied, enables one to increase the efficiency of utilization of energy which is put into the dryer at the compressor and to minimize the size of the compressor.

However, the second approach has several serious disadvantages. For one thing, it necessarily involves a gas-to-gas heat exchanger. Such exchangers are inherently bulky and thermally inefficient, because of the large heat exchange areas required to transfer a given amount of heat per unit time. This result flows from the low heat capacities of gases and from the poor coefficients of heat transfer for gases, as compared to liquids, or condensing or evaporating liquids.

Another difficulty with using the cold product gas to precool the incoming gas is that such an arrangement produces a rather inflexible drying system. Somewhat paradoxically, the loss of flexibility occurs on the side of less strenuous drying requirements. For example, if one desired to use a unit with a gas-to-gas precooling stage designed to produce a gas with a 35 F dew point to produce a somewhat wetter gas, such as one with a 60 F. dew point, in the hope of obtaining a commensurately larger volume of product gas, it would be found that the increase in capacity is disappointingly low, because the comparatively warmer dry gas from the drying stage is a relatively poorer heat transfer agent in the gas-togas precooling stage.

I have found that the foregoing disadvantages, and others, can be overcome, avoided, and eliminated by utilizing the cooled product gas which has been heat exchanged with the refrigerating fluid, not to precool incoming gas, but to cool and condense compressed refrigerating fluid, prior to its expansion and evaporation in the dryer stage unit. Since the cool gas issuing from the dryer stage of the unit is, in accordance with my invention, heat exchanged with a condensing liquid, namely, the refrigerating fluid, the heat exchangers structure for accomplishing this can be much smaller and simpler in construction than the bulky and inefficient gas-to-gas heat exchangers used heretofore to exploit the heat absorbing capacity of the cold dried gas. Furthermore by using the cold dried gas to condense part of the refrigerating fluid, in parallel with a conventional condenser using ambient air on the cold side to condense the rest of the refrigerating fluid in the circuit, l obtain not only the foregoing advantages but increased gas drying capacity, for a given size refrigeration system, increased freedom from the effects of ambient temperature on the system capacity, and increased system flexibility in that by accepting wetter product gas from a particular system I can obtain a commensurately higher volume of product gas.

By abandoning the prior art convention of utilizing product gas to precool input gas several other material advantages are obtained. First, the condensation of water from the incoming gas, which is the object of the whole operation, is carried out in a single exchanger, thus simplifying its collection and disposal. In addition, the overall size of the equipment, for a given capacity and product dew point, is much reduced.

In accordance with other aspects of the invention several important equipment design advantages are obtained. One such improvement is in the provision of coupled heat exchangers within a single enclosure, one of them being a gas-drying unit and the other being a condenser for the refrigerating fluid using the dried cool product gas as the coolant. Another such improvement is the provision of a specially structured heat exchanger for evaporating refrigerating fluid which minimizes objectionable accumulation of refrigerating fluid lubricant therein.

In accordance with still further aspects of the invention 1 have found that by placing an indicator of the kind which changes color at a selected relative humidity in the gas stream as it is being reheated following moisture removal, I obtain a useful control indicator for assessing the performance of the dryer stage.

It is an object of this invention to provide a method and apparatus for drying compressed gas by refrigeration, which method and apparatus have improved flexibility in capacity, increased drying capacity, and decreased sensitivity to ambient temperature, compared to prior systems.

A further object of this invention is to provide an apparatus for drying compressed gas by refrigerating it and separating the condensed water, which apparatus is smaller, simpler, and more efficient than those employed heretofore.

Another object of the invention is to provide an improved refrigerating fluid evaporator, especially useful in a compressed air-drying system, in which objectionable accumulation of refrigerating fluid lubricant is overcome.

Still another object of the invention is to provide a compressed gas-drying system having a simple and effective indicating means therein for measuring the performance of the dryer.

The above objects and purposes, together with other objects and purposes can best be understood by considering the following detailed description, together with the accompanying drawings in which:

FIG. 1 is an isometric view, somewhat simplified, of a preferred form of drying apparatus constructed in accordance with the invention;

FIG. 2 is a flow diagram for the dryer of FIG. 1;

FIG. 3 is a vertical sectional view, partly diagrammatic, of an alternate embodiment constructed in accordance with the invention, which embodiment is particularly useful for dryers having relatively small compressed gas flow rates; and

FIG. dis a fragmentary elevational view of a portion of the dryer of FIG. 3, illustrating additional features of the invention.

The general operation of the method and equipment of the invention can best be understood by considering first the flow diagram of lFlG. 2. The equipment shown in FIG. 2 includes a first heat exchanger or chiller MB, and a second heat exchanger or reheater illl. These are the principal pieces of equipment through which the gas to be dried is flowed, and in accordance with the invention it passes first through the chiller and then through the reheater. The gas handling portion of the equipment also includes a water separator and trap 112.

The refrigeration circuit of the equipment includes compressor 113, condenser 114i, condenser fan 11$, receiver M, and expansion valve 17. The various pieces of refrigerating equipment are interconnected with one another, and with the chiller lit and reheater 1111, by conduits to provide a sealed refrigeration system. Within the refrigeration system is a suitable refrigerant such as one of the common Freons.

As can be seen from FlG. 2, wet compressed gas is introduced to the unit through conduit llfl, which conducts it to chiller N). Here it is placed in heat exchange contact with the refrigerating fluid. Chiller MB is preferably a heat exchanger of the tube and shell type, with the compressed gas being on the shell side, and the refrigerating fluid on the tube side. in chiller 110 the wet compressed gas gives up heat to the refrigerating fluid, and in doing so vaporizes it. The wet gas thus falls in temperature below its dew point, and to a preselected, or design temperature well below the dew point of the incoming gas. As it does so, the water vapor in the gas condenses and falls out of the gas as liquid water. Since the compressed gas is in contact with liquid condensed water in the heat exchanger lli], it is substantially saturated throughout the exchanger at the temperatures prevailing at various points along the exchanger, but as the gas becomes cooler in its passage through the exchanger, it holds a smaller absolute amount of water (pounds per cubic foot). The cooled gas having a reduced moisture content then passes through conduit 119 into reheater ill. The condensed water also passes through conduit E9 and separator 2@ into trap 112. Water separator 20 and trap llZ may be any one of several well known types which are capable of collecting water in a pressurized system and discharging it intermittently or continuously from the system without permitting any substantial loss of gas from the system.

Reheater 1111 is also preferably a heat exchanger of the shell and tube type, and the compressed gas is conveniently passed through it on the shell side while the refrigerating fluid passes through it on the tube side. in this manner, the cooled compressed gas is brought into heat exchange contact with compressed hot refrigerating fluid. The refrigerating fluid gives up heat to the compressed gas, thus warming it and raising it to a temperature preferably well above its dew point. The compressed gas thus has a low absolute water content, and a low relative humidity at its outlet temperature. The reheated dry compressed gas leaves the unit through conduit 21 and is conducted to its point of use.

The flow and utilization of the refrigerating fluid in the method and equipment of FIG. 2 can best be understood by tracing the flow from receiver 116, through the unit, and back to receiver to. Liquid refrigerating fluid under pressure leaves receiver to through conduit 22 which conveys it to expansion valve 117. The expansion valve may be of any well-known type, including the capillary tube equivalent form. it is expanded through the valve and into the tube side of chiller llii through conduit 23. In chiller 110 the refrigerating fluid is heated by the compressed gas on the shell side and is vaporized. it passes out of chiller MD as a gas through conduit 24 which conveys it to compressor 113. Compressor 13 may conveniently be an integral motor-compressor in a hermetically sealed housing. The compressor increases the pressure on the refrigerating fluid gas, and the pressurized refrigerating fluid leaves the compressor through line 25.

Conduit 25 terminates at a distribution point 26. Part of the refrigerating fluid flows from the distribution point 26 into a conduit 27 which conveys it to the tube side of reheater ill. Here the refrigerating fluid gives up heat to the cool air on the shell side of reheater llll and thus condenses. The condensed refrigerating fluid leaves reheater M through conduit 2% which delivers it to receiver lid. The portion of the refrigerating fluid which did not pass through reheater llll passes from distribution point 26 through conduit 29 to condenser R4. Condenser lid may conveniently be a radiator type heat exchanger having finned tubes. Fan 15 forces ambient air past the finned tubes, and the refrigerating fluid in the tubes gives up heat to the air flowing over the tubes. The refrigerating fluid condenses and leaves condenser M through conduit 3d which delivers it to receiver lib.

With the foregoing description of the flow systems of FIG. 2 completed, certain observations can be made concerning the operating advantages obtained. The compressed refrigerating fluid arriving at distribution point 26 will divide itself into two streams through parallel heat exchangers 11 ll and lid on its way to receiver To. The quantities of refrigerating fluid flowing in each of the two streams will depend upon and vary with the relative condensing capacity of the two heat exchangers 111 and H4 at any given time. The condensing capacity of each exchanger is in turn a function of its heat flux at any given time, and as operating conditions, such as input gas temperature and humidity and ambient air temperature, vary, the heat flux in the heat exchangers will also vary. As the heat flux in one heat exchanger increases, relative to the heat flux in the other exchanger, the pressure in the first exchanger, on the refrigerating fluid side, will tend to drop. This will increase the pressure differential between the interior of the first heat exchanger and the distribution point 26 upstream. in response to this increase in pressure differential, there will be an increase in flow of refrigerating fluid from point 26 into the first heat exchanger, and a reduction in flow into the second. The flow into the first heat exchanger will increase until the pressure in that heat exchanger is equal to that in the second heat exchanger. When this condition is reached, the relative flow rates in the two parallel streams will be stabilized until there is another relative change in the heat fluxes of the two heat exchangers. in summary, the flow rates in the two streams will continuously adjust to maintain equal pressures in the two heat exchangers. in this manner the condensing load is automatically and continuously divided between the two heat exchangers in proportion to their relative ability to handle it. This self-division of the refrigerating fluid condensing load between heat exchangers 111 and 114i contributes greatly to making it possible to increase the capacity of the unit by accepting a somewhat wetter compressed gas product stream, if conditions make this desirable. Thus, if the flow rate of wet gas into chiller it) is increased, the gas leaving chiller lit and entering reheater llll will be warmer, and thus have less capacity to cool refrigerating fluid in reheater iii. The compressed gas entering reheater llli could even be above ambient temperature. But such a condition will not materially reduce the amount of liquid refrigerating fluid supplied to receiver To, since condenser M will make up the deficiency.

The parallel heat exchanger arrangement also improves the system '5 flexibility in handling input compressed gases of varying temperatures. Thus, assume that it is desired to dry the input compressed gas to a dew point a stated number of degrees below its input temperature. Assume further that the input temperature of the compressed gas rises so high that even after being chilled to the desired dew point, the gas in conduit 11% is above ambient temperature. Again, the compressed gas in reheater ill will be relatively disabled as a coolant for a refrigerating fluid, but once again, the ambient air operated condenser M will pick up the load and provide sufficient liquid refrigerant to the receiver to maintain the cycle.

Despite the increased flexibilities just explained, the system illustrated in FlG. 2 has a reduced dependence on variations in ambient temperatures as compared with prior systems. if the ambient temperature rises, for example, and the condenser M thus loses part of its capacity to liquify refrigerating fluid, the reheater ill will tend to pick up more of the liquification load and assure a continuing supply ofliquid to receiver lib.

Some variations in the invention can also be understood from a consideration of FIG. 2. Taking the compressed gas side of the system first, it should be noted that as a matter of flow path, separator 20 is positioned between chiller l0 and reheater 11. This is in fact a convenient location, but the separator can be located in other positions adjacent (in a flow path sense) chiller l0, and it may be desirable to so relocate it in order to take advantage of gravity flow to convey water to the separator. FIG. 2 shows a single chiller stage and shows it arranged for counter current flow of heat exchange fluids, but in accordance with heat exchange technology multiple chiller stages and/or concurrent flow may also be used. Similar comments apply with respect to reheater ll.

Turning to the refrigerating fluid side of the system it should be noted that FIG. 2 shows the use of a receiver 16, which provides a convenient inertia" to reduce the sensitivity of the refrigeration circuit "inertia"minor operating variations in various parts, such as the compressor. However, a separate receiver is not especially and liquid refrigerant can be passed directly from its point of liquification to the expansion valve. FIG. 2 shows a forced air condenser 14, but in small units for the sake of economy reliance can be placed on a convective ambient air-type condenser.

While it is preferred, especially in larger capacity units, to have the two heat exchangers which condense refrigerating fluid arranged-in parallel, as they are in FIG. 2, many of the advantages of the invention are retained if they are placed in series. The series arrangement means that all of the refrigerating fluid will pass through both of the heat exchangers, but the division of the liquifying load between the two exchangers will still be accomplished, at least to a large degree. An arrangement of the type just described is discussed in more detail in connection with FIG. 3.

Certain of the structural features of the invention are shown in FIG. 1, where reference characters corresponding to those used in FIG. 2 are employed insofar as possible. The apparatus shown in FIG. ll includes a frame 35 with the tube heat exchangers mounted on the top thereof, and the refrigerating equipment, including the forced air condenser mounted on a tray 36 in the lower part thereof. Thus, chiller appears toward the rear at the top of the unit, and it is provided with inlet conduit 18. Mounted beside it is reheater 11, also a cylindrical heat exchanger, equipped with outlet conduit 21. Chiller 10 and reheater 11 are connected by conduit 19 which extends from the bottom of chiller 10 hear one end to the bottom of reheater 11 at one end thereof. Separator depends from conduit 19. This arrangement has the advantage that separator 20 is positioned below both chiller l0 and reheater Ill and can gather not only the bulk of the water, which condenses in chiller 10, but also any water which condenses in conduit 19 connecting them.

As mentioned before, compressor 13, receiver 16, fan 15, and condenser 14 are mounted on tray 36. It is preferred that receiver 16 be mounted beneath the outlet of both reheater 11 and condenser 14, to exploit gravity in gathering condensed refrigerating fluid into it. Expansion valve 17 appears at the upper left-hand side of FIG. 1 adjacent the refrigerating fluid input to chiller 10. The various conduits completing the refrigeration circuit are numbered just as they are in FIG. 2, and small arrows are used to indicate flow direction.

While the chiller 10 illustrated in FIG. 1 is one having only a single tube serving as the tube side of the exchanger, it should be understood that the same basic construction can be used when the tube side is of the multiple tube type. Furthermore, in both single and multiple tube types the individual tubes may be formed into coils or trombone loops inside the chiller. Similarly, reheater 11 is shown as a single tube heat exchanger, but it may be built as a multitube type, with or without coiling or tromboning of the tubes within the shell.

Turning now to the alternate embodiment of FIG. 3, it can be seen that it too has a chiller 10a, a reheater lla, a gas inlet 18a, a water trap 12a, and a gas outlet 210, all being part of the compressed gas side of the system. On the refrigerating fluid side of the system, the unit of FIG. 3 has a compressor 130, an ambient air condenser 14a, and expansion means 17a, which are in the form of a coiled capillary tube. The unit shown in FIG. 3 has a number of constructional features which make it an advantageous system for use in situations where the flow of compressed gas is relatively low.

While chiller 10a and reheater 11a are functionally separate heat exchangers, they are contained within a single heat exchanger shell 45. Shell 45 is closed at its upper end by head piece 46. The head piece 46 has gas outlet fitting 21a threaded to it. Refrigerating fluid conduit 47 is passed through the hole 48 in the end of head 46 and is sealed in gastight manner by bulkhead fitting 49.

The other end of shell 45 is similarly closed by head piece 50 which has gas inlet 18a threaded into it. Head 50 also has a refrigerating fluid line 51 passing through it, and sealed in a gastight manner by bulkhead fitting 52. Conduit 53 is also threaded into head 50, and is attached at its other end to trap 12a.

As FIG. 3 shows, chiller 10a is positioned inside shell 45 beneath reheater 110, or, stated differently, shell 45 is oriented more or less vertically so that chiller 10a is below reheater lla. This arrangement, combined with the placement of the water trap below chiller 10a insures that water condensing in chiller 10a, is effectively separated by gravity flow and gathered in trap 12a. Chiller 10a and reheater are separated from one another within shell 45 by a piece of steel wool 53a or the like, which serves to deentrain mist which may be carried upwardly from chiller 10a, and thus functions as a water separator.

Reheater 11a comprises a tube 54 through which refrigerating fluid to be condensed passes in heat transfer contact with cooled dry gas flowing upwardly through shell 45. The tube is fitted with fins 55 extending into the shell space to increase the available heat transfer area. Expansion capillary 17a is attached to the bottom of tube 54 and condensed refrigerating fluid leaves tube 54 through the capillary.

The expansion capillary 17a is coiled in the shell space of chiller 10a, among other things, to conserve space while still obtaining the required length of capillary to enable it to function as a effective expansion means. It should be noted that an expansion valve may be employed in place of the capillary. The capillary 17a is connected to the bottom of tube 56 which forms the tube side of chiller l0a.Refrigerating fluid expands through the capillary and enters the bottom of tube 56 where it is presented in heat exchange contact with warm, humid gas flowing upwardly through chiller 10a. Tube 51 extends into heat exchange tube 56, as shown in FIG. 3, but terminates short of the upper end of tube 56. Refrigerating fluid which vaporizes in tube 56 passes into tube 51 which delivers it to compressor 13a. Chiller tube 56, like reheater tube 54, is preferably equipped with fins 57 to increase the available heat transfer surface.

The construction of tube 56 just described prevents undesirable accumulation of refrigerating fluid lubricant therein. The lubricants commonly mixed with refrigerating fluid are normally least soluble in the fluid at the point of lowest temperature. It is here that they will tend to accumulate, and to the extent they collect at such a point in the system, they are unable to perform lubricating functions at other points, as in the compressor.

The coldest point on the refrigeration side of FIG. 3 is near the bottom of tube 56, and thus lubricant will tend to separate from the refrigerating fluid, and form a liquid phase separate from the liquid phase of refrigerating fluid in the bottom of tube 56. However, as the refrigerating fluid vaporizes within tube 56, it passes upwardly to enter tube 51. In doing so, the upwardly moving vapors sweep a film of lubricant up the outside surface of tube 51, and ultimately into the interior of tube 51. In this manner, the separated lubricant at the bottom of tube 56 is continuously reintroduced into the refrigerating fluid flow stream, and thus does not objectionably accumulate.

in the embodiment of FIG. 3 the condenser Ma is a simple finned tube, depending upon convection to move ambient air past it. However, if desired, other forms of condenser, including fan driven condenser can be used. A refrigerating fluid bypass line 5% is connected between tubes 47 and 51 and is equipped with valve 59. it serves as a means to shunt refrigerating fluid around the heat exchangers, thus varying the capacity of the system. An alternative means for the same purpose is a compressor capacity varying device, such as an on-off control.

The flow paths through the unit of FIG. 3 can now be briefly outlined, and attention is directed to the small arrows on FIG. 3 indicating flow directions. Compressed gas enters the unit through inlet ma and passes through head 52 into the shell side of chiller Mia. it passes upwardly through chiller Mia, cooling and losing water as it does so. The cool gas passes through mist separating means 53a and into the shell side of reheater Illa. it passes upwardly through reheater Illla, becoming warmer in passage, and leaves the unit through head as and outlet Zlla. Water separating from the gas falls downwardly through the shell side of unit into head 50, and thence through conduit 53 to trap 32a.

Refrigerating fluid is compressed in compressor 113a and delivered to condenser Ma where it gives up heat to ambient air. Little or even none of the refrigerating fluid may condense in heat exchanger Ma, but by losing heat it approaches the condensation point. The refrigeration fluid then passes through line 4'7 into the top of reheater Illa and condenses within tube 54 of the reheater. The liquified refrigerating fluid passes out of tube 543 and into capillary ll7a, where it is ex panded and delivered to tube 56 of chiller 1100. Here the refrigerating fluid vaporizes and leaves tube 56 through tube 511 which delivers it to compressor 130. From the foregoing it can be seen that the unit of Fit]. 3 is one in which the two heat exchangers Ma and Ma are connected in series on the refrigerating fluid side, rather than in parallel, as the corresponding heat exchangers of FIGS. l and 2 are.

lFllG. t shows in elevation the upper portion of the unit of Fit]. 3. As can be seen in FIG. 4, the shell 55 is formed of transparent material. While the transparency of the shell is important for the purposes of the invention only in the region of reheater lllla, it is convenient to make it of transparent material throughout its length. Positioned inside shell 35, and running along the length of reheater Ella is a strip of indicator material tit). Indicator Ml may conveniently be formed of a strip of paper impregnated with a chemical which changes color within a narrow humidity range, such as cobalt chloride or cobalt bromide. The particular indicator employed is a matter of choice, and the form of the indicator material may also be varied, as by distributing granules of indicator material along the tins of reheater lllla throughout most or all of its length.

From the foregoing, it can be seen that the indicator extends along a substantial portion of the length of the reheater Illa on the compressed gas side thereof. Cool compressed gas enters the reheater with a relative humidity of substantially 100 percent. As it warms during its flow through the reheater its relative humidity falls progressively, reaching its lowest level at the outlet end of the reheater, where it is warmest. in accordance with the invention an indicator is used which changes color at a relative humidity lower than 100 percent and higher than the designed output gas humidity. With an indicator so selected, there will be a point somewhere along the length of the reheater where the gas will have a relative humidity equal to that at which the indicator changes color. At such point a line of color demarcation will form on the strip of indicator material. Such a line is indicated at 61 on strip 60 in H0. 41.

This line of color demarcation l have found to be a useful control and monitoring means for the dryer. As the product gas becomes wetter, the line will move upwardly toward the output end of the reheater. As the product gas becomes drier, the line will move toward the input end of the reheater. For a particular unit the position of the line can be correlated with the relative humidity of the product gas, and after such calibration, separate measurements of relative humidity are unnecessary, since the operator can read the humidity directly from the color indicator. When the operator notes that the position of the color demarcation line has changed, he is informed that the relative humidity has changed, and can determine, again by reference to a calibration scale, what the new relative humidity is. if the change is in an undesired direction, the operator can take steps, such as changing the flow rate, to return the relative humidity to the desired level.

Iclaim:

ll. Dryer apparatus for compressed gas of the refrigerated type comprising a first heat exchanger adapted to present compressed gas to be dried and refrigerating fluid to be evaporated in heat transfer relationship, water separator means mounted adjacent said first heat exchanger adapted to collect water condensing from said compressed gas in said first heat exchanger and to discharge said water from contact with the gas while preventing substantial loss of gas, a second heat exchanger adapted to present relatively dry cool compressed gas and refrigerating fluid to be condensed in heat transfer relationship, a third heat exchanger adapted to present ambient air and refrigerating fluid to be condensed in heat transfer relationship, compressor means for said refrigerating fluid, receiver means for said refrigerating fluid, expansion means for said refrigerating fluid, gas conducting means establishing a flow path for compressed gas from an inlet point through said first heat exchanger and then through said second heat exchanger, to an outlet point, and conduit means for said refrigerating fluid connecting said receiver, expansion means, flrst heat exchanger and compressor in series, and further connecting said second and third heat exchangers in parallel between said compressor and said receiver, said conduit means including distribution means providing for simultaneous delivery of said refrigerating fluid to said second and third heat exchangers.

2. Dryer apparatus for compressed gas of the refrigerated type comprising a first heat exchanger adapted to present compressed gas to be dried and refrigerating fluid to be evaporated in heat transfer relationship, water separator means mounted adjacent said first heat exchanger adapted to collect water condensing from said compressed gas in said first heat exchanger and to discharge said water from contact with the gas while preventing substantial loss of gas, a second heat exchanger adapted to present relatively dry cool compressed gas and refrigerating fluid to be condensed in heat transfer relationship, a third heat exchanger adapted to present ambient air and refrigerating fluid to be condensed in heat transfer relationship, compressor means for said refrigerating fluid, expansion means for said refrigerating fluid, gas conducting means establishing a flow path for compressed gas from an inlet point through said first heat exchanger, and then through said second heat exchanger, to an outlet point, and conduit means for said refrigerating fluid connecting said expansion means, first heat exchanger and compressor in series, and further connecting said second and third heat exchangers between said compressor and said expansion means.

3. Dryer apparatus for compressed gas of the refrigerated type comprising a first heat exchanger adapted to present compressed gas to be dried and refrigerating fluid to be evaporated in heat transfer relationship, water separator means mounted adjacent said first heat exchanger adapted to collect water condensing from said compressed gas in said first heat exchanger and to discharge said water from contact with the gas while preventing substantial loss of gas, a second heat exchanger adapted to present relatively dry cool compressed gas and refrigerating fluid to be condensed in heat transfer relationship, a third heat exchanger adapted to present ambient air and refrigerating fluid to be condensed in heat transfer relationship, gas conducting means establishing a flow path for compressed gas from an inlet point through said first heat exchanger and then said second heat exchanger, to an outlet point, and a closed circuit compression-condensationexpansion'vaporization refrigeration fluid system including conduit means delivering refrigerating fluid to be evaporated to said first heat exchanger and refrigerating fluid to be condensed to said second and third heat exchangers.

4. Apparatus according to claim 3 in which said conduit means deliver refrigerating fluid to be condensed in parallel flow paths to said second and third heat exchangers, said conduit means including distribution means providing for simultaneous delivery of said refrigerating fluid to said second and third heat exchangers.

5. Apparatus according to claim 3 in which said conduit means deliver refrigerating fluid to be condensed first to said third heat exchanger and then to said second heat exchanger.

6. Apparatus according to claim 3 further comprising a transparent wall for said second heat exchanger rendering the gas side thereof visible to an external observer for a substantial portion of the gas flow path through said second heat exchanger, and indicating means changing color at a selected relative humidity positioned in said second heat exchanger on the gas side thereof in a location visible through said transparent wall, said indicating means extending along a substantial portion of the gas flow path through said second heat exchanger.

7. Apparatus according to claim 3 in which said first and second heat exchangers are of the tube and shell type, and in which the compressed gas flows therethrough on the shell side, and further comprising a common generally cylindrical shell for said first and second heat exchangers, said shell being oriented to position said first heat exchanger at a level lower than the second, said water separator means including trap means mounted below said first heat exchanger, and said refrigeration system including refrigerating fluid expansion means positioned within said shell and connecting said second and first heat exchangers.

8. A method for drying compressed gas by refrigerating it through the use of a closed circuit compression-condensationexpansion-vaporization refrigeration fluid system comprising heat exchanging gas to be dried against liquid refrigerating fluid, thereby cooling the gas and condensing water from it,

and evaporating refrigerating fluid, separating the condensed water from contact with the gas, collecting and compressing the evaporated refrigerating fluid, heat exchanging cooled gas with at least part of the compressed refrigerating fluid, thereby warming the gas and reducing its relative humidity, and condensing compressed refrigerating fluid, concurrently heat exchanging ambient air with at least part of the compressed refrigerating fluid, and removing warmed and dried gas to a point of use.

9. A method according to claim 8 in which said compressed refrigerating fluid is heat exchanged with cooled gas and with ambient air in parallel streams, and in which at least part of said compressed refrigerating fluid is condensed by heat exchange with ambient air.

10. A method for drying compressed gas by refrigerating it through the use of a closed circuit compression-condensationexpansion-vaporization refrigeration fluid system comprising heat exchanging gas to be dried against liquid refrigerating fluid, thereby cooling the gas and condensing water from it, and evaporating refrigerating fluid, separating the condensed water from contact with the gas, collecting and compressing the evaporated refrigerating fluid, heat exchanging cooled gas with at least part of the compressed refrigerating fluid, thereby warming the gas and reducing its relative humidity, and condensing compressed refrigerating fluid, heat exchanging ambient air with at least part of the compressed refrigerating fluid, removing warmed and dried gas to a point of use and exposing said gas during the warming thereof to an indicator changing color at a selected relative humidity extending along the path of flow of the gas being warmed, thereby obtaining a measure of the relative humidity of the warmed and dried gas.

11. in the method of drying compressed gas by cooling it to a temperature below its dew point and reheating it along a flow path to a use temperature, the improvement which comprises exposing said gas during the reheating thereof to an indicator changing color at a selected relative humidity distributed along said flow path thereby establishing a line of color demarcation in said indicator the position of which along said flow path is a function of the relative humidity of said gas at its use temperature. 

2. Dryer apparatus for compressed gas of the refrigerated type comprising a first heat exchanger adapted to present compressed gas to be dried and refrigerating fluid to be evaporated in heat transfer relationship, water separator means mounted adjacent said first heat exchanger adapted to collect water condensing from said compressed gas in said first heat exchanger and to discharge said water from contact with the gas while preventing substantial loss of gas, a second heat exchanger adapted to present relatively dry cool compressed gas and refrigerating fluid to be condensed in heat transfer relationship, a third heat exchanger adapted to present ambient air and refrigerating fluid to be condensed in heat transfer relationship, compressor means for said refrigerating fluid, expansion means for said refrigerating fluid, gas conducting means establishing a flow path for compressed gas from an inlet point through said first heat exchanger, and then through said second heat exchanger, to an outlet point, and conduit means for said refrigerating fluid connecting said expansion means, first heat exchanger and compressor in series, and further connecting said second and third heat exchangers between said compressor and said expansion means.
 3. Dryer apparatus for compressed gas of the refrigerated type comprising a first heat exchanger adapted to present compressed gas to be dried and refrigerating fluid to be evaporated in heat transfer relationship, water separator means mounted adjacent said first heat exchanger adapted to collect water condensing from said compressed gas in said first heat exchanger and to discharge said water from contact with the gas while preventing substantial loss of gas, a second heat exchanger adapted to present relatively dry cool compressed gas and refrigerating fluid to be condensed in heat transfer relationship, a third heat exchanger adapted to present ambient air and refrigerating fluid to be condensed in heat transfer relationship, gas conducting means establishing a flow path for compressed gas from an inlet point through said first heat exchanger and then said second heat exchanger, to an outlet point, and a closed circuit compression-condensation-expansion-vaporization refrigeration fluid system including conduit means delivering refrigerating fluid to be evaporated to said first heat exchanger and refrigerating fluid to be condensed to said secoNd and third heat exchangers.
 4. Apparatus according to claim 3 in which said conduit means deliver refrigerating fluid to be condensed in parallel flow paths to said second and third heat exchangers, said conduit means including distribution means providing for simultaneous delivery of said refrigerating fluid to said second and third heat exchangers.
 5. Apparatus according to claim 3 in which said conduit means deliver refrigerating fluid to be condensed first to said third heat exchanger and then to said second heat exchanger.
 6. Apparatus according to claim 3 further comprising a transparent wall for said second heat exchanger rendering the gas side thereof visible to an external observer for a substantial portion of the gas flow path through said second heat exchanger, and indicating means changing color at a selected relative humidity positioned in said second heat exchanger on the gas side thereof in a location visible through said transparent wall, said indicating means extending along a substantial portion of the gas flow path through said second heat exchanger.
 7. Apparatus according to claim 3 in which said first and second heat exchangers are of the tube and shell type, and in which the compressed gas flows therethrough on the shell side, and further comprising a common generally cylindrical shell for said first and second heat exchangers, said shell being oriented to position said first heat exchanger at a level lower than the second, said water separator means including trap means mounted below said first heat exchanger, and said refrigeration system including refrigerating fluid expansion means positioned within said shell and connecting said second and first heat exchangers.
 8. A method for drying compressed gas by refrigerating it through the use of a closed circuit compression-condensation-expansion-vaporization refrigeration fluid system comprising heat exchanging gas to be dried against liquid refrigerating fluid, thereby cooling the gas and condensing water from it, and evaporating refrigerating fluid, separating the condensed water from contact with the gas, collecting and compressing the evaporated refrigerating fluid, heat exchanging cooled gas with at least part of the compressed refrigerating fluid, thereby warming the gas and reducing its relative humidity, and condensing compressed refrigerating fluid, concurrently heat exchanging ambient air with at least part of the compressed refrigerating fluid, and removing warmed and dried gas to a point of use.
 9. A method according to claim 8 in which said compressed refrigerating fluid is heat exchanged with cooled gas and with ambient air in parallel streams, and in which at least part of said compressed refrigerating fluid is condensed by heat exchange with ambient air.
 10. A method for drying compressed gas by refrigerating it through the use of a closed circuit compression-condensation-expansion-vaporization refrigeration fluid system comprising heat exchanging gas to be dried against liquid refrigerating fluid, thereby cooling the gas and condensing water from it, and evaporating refrigerating fluid, separating the condensed water from contact with the gas, collecting and compressing the evaporated refrigerating fluid, heat exchanging cooled gas with at least part of the compressed refrigerating fluid, thereby warming the gas and reducing its relative humidity, and condensing compressed refrigerating fluid, heat exchanging ambient air with at least part of the compressed refrigerating fluid, removing warmed and dried gas to a point of use and exposing said gas during the warming thereof to an indicator changing color at a selected relative humidity extending along the path of flow of the gas being warmed, thereby obtaining a measure of the relative humidity of the warmed and dried gas.
 11. In the method of drying compressed gas by cooling it to a temperature below its dew point and reheating it along a flow path to a use temperature, the improvement which comprises exposing saId gas during the reheating thereof to an indicator changing color at a selected relative humidity distributed along said flow path thereby establishing a line of color demarcation in said indicator the position of which along said flow path is a function of the relative humidity of said gas at its use temperature. 